1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly to an image forming apparatus for obtaining an image by transferring a visible image (toner image), formed on an image bearing member by an electrophotographic process or an electrostatic recording process, onto a transfer material supported on a transfer material support member of a transfer device. Such image forming apparatuses include black-and-white, monochromatic or full-color electrophotographic copying machines, printers and other recording equipment.
2. Related Background Art
Among various image forming apparatuses of the electrophotographic or electrostatic recording type, there is already known a color electrophotographic copying apparatus capable of copying a full-color image, as disclosed in Japanese Patent Appln. Laid-Open No. 55-32079. Said apparatus transfers color toner images, formed on a photosensitive drum constituting the image bearing member, one by one in superposed manner onto a sheet-shaped transfer material supported on a transfer drum constituting the transfer material support means, by means of a transfer charger. A known structure of the transfer drum consists of two mutually opposed ring portions, a connecting portion for said ring portions, and a transfer material supporting sheet covering an aperture formed by said ring portions and connecting portion.
Such transfer drum is designed so that the peripheral length thereof can sufficiently accommodate the transfer sheet of the maximum size to be used, and the size of the photosensitive drum is selected so that the ratio of the peripheral length of the photosensitive drum to that of the transfer drum is an integer (L2/L1 or L1/L2 is an integer, wherein L1 is the peripheral length of the photosensitive drum, while L2 is that of the transfer drum). For example, if the transfer drum has a diameter of 160 mm, the diameter of the photosensitive drum is selected as 80 or 160 mm. Though the periodical fluctuation in the load of the photosensitive drum or the transfer drum is a major cause of aberration in colors because of the fluctuation in the rotating speed, such aberration in colors rarely appears in the case of the above-mentioned integral ratio, because the extension or contraction takes place similarly for each of the in different colors. If such integral ratio is not adopted, then the aberration in colors because of the above-mentioned cause is unavoidable, and it is extremely difficult to reduce such aberration in colors even with improvements in the driving means.
However, such integral ratio between the diameters of the photosensitive drum and the transfer drum gives rise to a shortened service life of the photosensitive drum, because of the following reasons:
(1) the transfer sheet, the connecting portion and the leading end of the transfer material supporting sheet impinge repeatedly on same positions of the photosensitive drum, thereby causing damage thereto; and PA1 (2) charging by the transfer charger takes place strongly or weakly, depending on the position, such as the leading or trailing end of the transfer sheet or the connection portion, and repetition of such charging in the same position causes a memory on the photosensitive drum (charge memory).
These drawbacks are naturally encountered also when the above-mentioned transfer drum is replaced by a transfer belt not equipped with the ring portions or the connecting portion.